Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to obtain higher levels of pollution abatement and increased fuel economy. Among the known options, direct fuel injection appears to be one of the best candidates for achieving improved performance but higher initial costs have tended to discourage its general adoption. This situation is accentuated because only the more sophisticated and more expensive direct injection systems are capable of achieving the increasingly higher performance goals of engine manufacturers.
Until recently, attempts to provide a low cost fuel injection system have tended to center on distributor type fuel injection systems having a single centralized high pressure pump and a distributor valve for metering and timing fuel flow from the pump to each of a plurality of injection nozzles, such as disclosed in U.S. Pat. No. 3,557,765. Although simple design concept, systems of this type generally suffer defects inherent with separation of the injector nozzles from the centralized pump. Unit injector systems avoid these inherent defects by providing each engine cylinder with its own cam-actuated pump such as disclosed in U.S. Pat. No. 3,544,008. Nevertheless, the performance advantage of unit injectors has generally not outweighed the detriment of greater costs except for heavy duty compression ignition engine applications. The design of a commercially competitive unit fuel injector therefore normally involves acceptance of some characteristics which are less than optimal since the basic goals of low cost, high performance and reliability are often in direct conflict.
Additionally, as the need for higher engine efficiency and pollution abatement have increased, it has become increasingly evident that some economical means must be provided to vary injector timing in response to changing engine operating conditions. Such control is relatively straightforward in distributor-type fuel injector systems since the injection event is controlled at one central location. However, in unit injector systems, control over injector timing ordinarily requires modification of each individual unit injector, thereby adding significantly to the overall cost of the system.
U.S. Pat. Nos. 2,997,994 and 2,863,438 provide examples of attempts to solve this dilemma by disclosing a fairly simple mechanism for achieving variable timing in unit injectors. In particular, these patents disclose the use of a collapsible hydraulic link to selectively change the effective length of the cam operated fuel injector plunger. However, the simplicity of these hydraulic timing controls is achieved only by operating the hydraulic link in either a fully expanded or fully collapsed mode. Thus, there can only be a stepped change in timing of the injection event which will not necessarily suit the broad range of conditions normally encountered during the operation of an engine. Attempts to provide for infinite variations in injection timing, even when a hydraulic link is employed, have generally involved the use of a mechanical rack which controls the size and/or the point of collapse of the hydraulic link as disclosed in U.S. Pat. Nos. 3,847,510 and 4,092,964.
Examples of techniques for providing infinite variation of unit injector timing by other means are illustrated in U.S. Pat. Nos. 3,035,523 and 3,083,912 which disclose fairly complex hydraulic arrangements for this purpose. However, in these systems the quantity injected and the change in timing are interrelated and may not be controlled independently of one another.
In U.S. Pat. No. 4,410,138 to Peters, commonly assigned to the assignee of this application, Cummins Engine Company, Inc., a fuel injector having infinitely variable timing occurring throughout the entire range of possible injection rates is disclosed. This injector uses a two part injector plunger which forms a variable length timing chamber between the upper and lower plungers and a stepped spring biasing force which resists timing chamber expansion. This injector is arranged to provide pressure and time metering of fuel or fuel cur off. U.S. Pat. No. 4,463,901, coinvented by Dr. Perr, one of the coinventors of the present invention, and commonly assigned to the assignee of the present invention, also discloses a fuel injector having infinitely variable timing advance using a variable length hydraulic link. However, neither of these injectors permits a first stepped timing advance followed by a second infinitely variable timing advance.